1. Field of the Invention
This invention relates to a recording medium reproducing device such as a video tape recorder (which is hereinafter referred to as a VTR), and more particularly to a recording speed detecting device for detecting the driving speed of the recording medium in the recording operation.
2. Description of the Related Art
In a general VTR for home use, the helical scanning recording-reproducing system employing a rotating magnetic head is used. A cassette type tape is used as the magnetic tape for the VTR. The VTR can be selectively set to two or more different recording modes in which the same tape can be used over different periods of recording time. For example, in the VHS type VTR, three modes including SP (standard play) mode, LP (long play) mode and EP (extra play) mode can be selectively set.
In such a VTR, in order to obtain a normal picture in the reproduction operation, it is necessary to coincide the tape driving speed in the reproducing operation (which is hereinafter referred to as the reproduction speed) with the tape driving speed in the recording operation (which is hereinafter referred to as the recording speed). It would be troublesome for the operator to set the reproduction speed in coincidence with the recording speed, and therefore various recording speed determining devices which can automatically determine the recording speed of the magnetic tape and set the reproduction speed to the corresponding value have been provided.
One of the methods for automatically determining the recording speed of the magnetic tape is disclosed in Japanese Patent Disclosure (Kokai) 60-231,940. The method disclosed in the Japanese Patent Disclosure is explained below. First, a control pulse signal of a predetermined frequency is recorded on the magnetic tape. Next, the ratio of the reproduction output frequency of the pulse signal and the frequency of a rotation frequency signal of the capstan motor in the reproduction mode is detected. Then, the result of detection is compared with two reference values at least three times, and the results of comparison effected at least three times are time sequentially recorded. Finally, the recording speed of the magnetic tape is determined by the majority of the recorded results.
However, in order to exactly determine the recording speed based on the determination method described above, it is necessary to record at least three results of detection of the recording speed. For this reason, at least three stage storing circuits such as shift registers are necessary and time corresponding to that for determining the recording speed at the three stages is required in order to determine the recording speed of the magnetic tape. Of course, this is not preferable from the standpoint of reduction in size and high speed operation of the VTR. Therefore, it has been required to develop a recording speed determination device which can be formed of less constituents and determine the recording speed in a short period of time.
When, for example, three-stage shift registers are used, an erroneous determination may be made under certain conditions. For example, when erroneous data is extracted at every other stage, determining sections at the first and third stages may make an erroneous determination. Therefore, a determination error will occur despite the use of majority rule in order to reduce the possibility of error determination.